1. Field of the Invention
This invention relates to an electronic flash unit comprising an insulated gate bipolar transistor (abbreviated hereinafter as an I.G.B.T.) which is connected in series with a flash discharge tube and acts to control the emission of light from the flash tube, and more particularly to an electronic flash unit including a novel drive control system for the I.G.B.T.
2. Description of the Related Art
As one of electronic flash units using such an I.G.B.T., that disclosed in U.S. Pat. No. 4,839,686 is well known in the art.
As shown in FIG. 5, this known electronic flash unit is composed of a high-voltage DC power source 1 in the form of a known DC-DC converter circuit, a main capacitor 2 charged by the power source 1, a constant voltage circuit 3 associated with the power source 1 so as to supply a constant voltage to a light-emission control circuit 7 described later, a known trigger circuit 4 for triggering a flash tube 5, a control circuit 6 which is connected to control means 8 incorporated in a camera body to transmit and receive various signals between them thereby producing various output signals including a trigger signal for operating the trigger circuit 4, a light-emission control circuit 7 for controlling the on-off of the I.G.B.T. connected in series with the flash tube 5 thereby controlling the emission of light from the flash tube 5, and a double voltage circuit 9 for applying a doubled voltage to the flash tube 5.
In operation, when a switch Sw is turned on, the high-voltage DC power source 1 operates so that both the main capacitor 2 and a double voltage capacitor 9a are charged in the illustrated polarities. At the same time, a low-voltage DC power source E charges a power supply capacitor C for the control circuit 6, and the high-voltage DC power source 1 charges also a capacitor 3a in the constant voltage circuit 3.
When a light-emission starting command signal is applied to the control circuit 6 from the control means 8 in the camera body under the condition where the individual capacitors have been fully charged, a high-level trigger signal is generated from one output terminal O.sub.a of the control circuit 6 for a predetermined period in which the longest light emitting period of the flash tube 5 is considered.
As a result, transistors Q.sub.a and Q.sub.b in the light-emission control circuit 7 are turned on, and the voltage charged in the capacitor 3a is applied to the gate of the I.G.B.T. thereby turning on the I.G.B.T. At this time, the other output terminal O.sub.b of the control circuit 6 is maintained at a low potential level, so that a transistor Q.sub.c in the light-emission control circuit 7 is in its off state.
As soon as the I.G.B.T. is turned on, the known trigger circuit 4 operates to trigger or excite the flash tube 5, and, at the same time, the double voltage capacitor 9a is grounded at the plus (+) side through a resistor R1 and the I.G.B.T. The charged voltage of the double voltage capacitor 9a is superposed on that of the main capacitor 2, and the resultant voltage is applied to the flash tube 5.
As a result, the flash tube 5 emits light by consuming the charged energy of the main capacitor 2.
When, in the course of the emission of light from the flash tube 5, a light-emission stopping command pulse is applied to the control circuit 6 from, for example, a photometer circuit incorporated in the control means 8, a high-level light-emission stopping command signal is generated from the output terminal O.sub.b of the control circuit 6 thereby turning on the transistors Q.sub.c and Q.sub.d in the light-emission control circuit 7.
Due to the turning-on of the transistors Q.sub.c and Q.sub.d, the transistor Q.sub.a is short-circuited between its base and emitter, and the I.G.B.T. is also short-circuited between its gate and emitter, with the result that these transistors are turned off. Thus, the transistor Q.sub.b is also turned off. Therefore, the flash tube 5 stops the emission of light.
The operation described above is the basic operation of the prior art electronic flash unit shown in FIG. 5. The electronic flash unit is advantageous in that it operates without excess emission of light as compared to that using a terminating capacitor for stopping the emission of light. The electronic flash unit is also advantageous in that it can repeatedly emit light at a high speed, and it has a reduced size.
However, the I.G.B.T. turning-off system of the prior art electronic flash unit shown in FIG. 5 is such that a high-level light-emission stopping command signal from the output terminal O.sub.b of the control circuit 6 is applied to the system to turn off the I.G.B.T.
When the condition required for the high-level light-emission stopping command signal is considered, it is apparent that such a high-level signal must be continuously generated until the I.G.B.T. is completely turned off.
Thus, the control circuit 6 is required to include a so-called pulse generator circuit which generates such a high-level signal having a predetermined pulse width. Therefore, the prior art electronic flash unit is disadvantageous from the aspect of cost and also from the aspect of energy consumption.